The study is a prospective study of bone marrow chromosome changes in preleukemia. We are studying patients with refractory cytopenias and nondiagnostic bone marrow aspirates by direct bone marrow chromosome studies. All suspected preleukemic patients are followed by serial clinical, laboratory, and cytogenetic studies. Data collected on these patients is entered into a computer to be correlated with the course of the patient. The type and incidence of chromosome abnormalities will be determined in the preleukemic patients and the presence of such abnormalities related to the development of acute leukemia. The computer will be used to determine whether any clinical, laboratory or cytogenetic data have potential value as predictors of progression from preleukemia to leukemia. Consecutive patients with suspected preleukemia will be studied until 100 patients with chromosome abnormalities are collected; 273 patients with suspected preleukemia have been studied by chromosome analyses, 146 patients with overt acute leukemia and 151 "normal" subjects have been studied to date. The normal subjects provide data on normal variations in the normal human marrow karyotype, and are used as a basis for deciding normalacy or abnormality in the study patients.